There is a requirement in the bill to have an attendant at each station:

She noted a complaint-based enforcement process written in the bill that places "the onus on impacted individuals" to report violations and an unclear enforcement process once a complaint is made. Kotek added that widespread violation of the required attendant provision would undermine the intent of the bill.

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In Washington State they used to have state run liquor stores, so unlike many or most other states, you could only buy beer and wine in grocery stores. Costco wanted to sell all of it so lobbied hard to get the state liquor stores closed. I wouldn't be surprised if Costco was behind this move also. They want to do what they want to do.